Lighting from area sources, soft shadows, and interreflections are important effects in realistic image synthesis. Unfortunately, general methods for integrating over large-scale lighting environments, including Monte Carlo ray tracing, radiosity, or multi-pass rendering that sums over multiple point light sources, are impractical for real-time rendering. (Monte Carlo ray tracing is described by, inter alia, Cook, R, Porter, T, and Carpenter, L, Distributed Ray Tracing, SIGGRAPH '84, 137-146; Jensen, H, Global Illumination using Photon Maps, Eurographics Workshop on Rendering 1996, 21-30; and Kajiya, J, The Rendering Equation, SIGGRAPH '86, 143-150.) (A radiosity technique is described in Cohen, M, and Wallace, J, Radiosity and Realistic Image Synthesis, Academic Press Professional, Cambridge, 1993.) (Various multi-pass rendering techniques are described by, inter alia, Haeberli, P, and Akeley, K, The Accumulation Buffer: Hardware Support for High-Quality Rendering, SIGGRAPH '90, 309-318; Keller, A, Instant Radiosity, SIGGRAPH '97, 49-56; and Segal, M, Korobkin, C, van Widenfelt, R, Foran, J, and Haeberli, P, Fast Shadows and Lighting Effects Using Texture Mapping, SIGGRAPH '92, 249-252.)
Real-time, realistic global illumination encounters three difficulties—it must model the complex, spatially-varying bi-directional reflectance distribution functions (BRDFs) of real materials (BRDF complexity), it requires integration over the hemisphere of lighting directions at each point (light integration), and it must account for bouncing/occlusion effects, like shadows, due to intervening matter along light paths from sources to receivers (light transport complexity). Much research has focused on extending BRDF complexity (e.g., glossy and anisotropic reflections), solving the light integration problem by representing incident lighting as a sum of directions or points. Light integration thus tractably reduces to sampling an analytic or tabulated BRDF at a few points, but becomes intractable for large light sources. A second line of research samples radiance and pre-convolves it with kernels of various sizes. (See, e.g., Cabral, B, Olano, M, and Nemec, P, Reflection Space Image Based Rendering, SIGGRAPH '99, 165-170; Greene, N, Environment Mapping and Other applications of World Projections, IEEE CG&A, 6(11):21-29, 1986; Heidrich, W, Seidel H, Realistic, Hardware-Accelerated Shading and Lighting, SIGGRAPH '99, 171-178; Kautz, J, Vazquez, P, Heidrich, W, and Seidel, H, A Unified Approach to Pre-filtered Environment Maps, Eurographics Workshop on Rendering 2000, 185-196; and Ramamoorthi, R, and Hanrahan, P, An Efficient Representation for Irradiance Environment Maps, SIGGRAPH '01, 497-500.) This solves the light integration problem but ignores light transport complexities like shadows since the convolution assumes the incident radiance is unoccluded and unscattered. Finally, clever techniques exist to extend complexity of light transport, especially shadows. Light integration becomes the problem; almost all these techniques are unsuitable for very large light sources.